Air purification device

ABSTRACT

An apparatus for purifying air comprising two electrodes having a dielectric material such as glass extending therebetween. The dielectric material is air permeable, for example, in the form of a bed of discrete particles such as glass beads. The electrodes are also air permeable, allowing the air to flow through the electrodes and dielectric. Ozone is generated by discharge at points of contact of the dielectric particles. Airflow through the device is improved, allowing greater cooling, and higher volumes of airflow but at lower concentrations of ozone production than with prior art devices, thus reducing toxicity.

This is a 371 of PCT/GB99/03006, filed Sep. 9, 1999, which claimspriority from GB 9819540.7, filed Sep. 9, 1998.

This invention relates to an air purification device, and particularlybut not exclusively to an air purification device which generates ozone.

By air purification device is meant a device which may be used toimprove the quality of air, by removal of at least a portion ofpollutants, odours, airborne particles, bacteria and/or the like fromthe air. Such a device may be used to clean exhaust gases from motorvehicles and other polluting processes, for sterilization purposes in“clean” manufacturing facilities, hospitals, food storage andpreparation areas, etc.

It is known that the introduction of ozone into air, typically byelectrical generation of ozone, may be used as a way of purifying theair. Ozone is poisonous to bacteria and microorganisms, and alsooxidises many organic compounds that may be present in odours, howeveras ozone is also toxic to humans, its use as an air purifier has hadlimited application.

Conventionally, ozone generators are operated using the “silentdischarge” or “dielectric barrier principle” to convert oxygen intoozone. In such generators, the feedstock is typically oxygen gas becausethe use of air in a conventional device can lead to the production ofundesirable end products such as oxides of nitrogen (e.g. NO_(x)). Theuse of air as a feedstock can however be achieved using generators ofthe dielectric barrier type provided that they are operated at a highfrequency. Such high frequency devices comprise two electrodes having agap between them, air being passed through the gap so that electricaldischarge across the electrodes causes the oxygen in the air to beconverted to ozone. These devices generally have very narrow gapsbetween the electrodes and thus offer considerable resistance to theflow of air through the device, requiring a pump to provide airflow.

When such devices are used in air purification applications, theygenerally have to be left running for a long period of time (e.g.overnight) so that enough air can pass through the electrodes to beconverted to ozone. The concentrations of ozone produced can be quitehigh and it is therefore necessary to prevent access to the area that isbeing purified. Conventional devices can thus only be used in confinedspaces that can be controlled easily.

GB-A-2296172 discloses an ozone generator having dielectric spacersbetween the electrodes. The spacers maintain the spacing between theelectrodes, which is important as the pressure between the electrodescan become raised above atmospheric pressure, causing the electrodes tobow or warp. The gap between the electrodes is of the order of a fewmillimeters or less, to help reduce the voltage required to establish acorona discharge. In some embodiments, an open dielectric foam, adielectric filament material or irregularly spaced dielectric particlesare placed in the electrode gap around the spacers, to provide animpedance to the flow of gas through the electrode gap so that gas flowbetween the electrodes and hence corona discharge is uniform. The airflow through the device is not high enough to enable the apparatus to beused as an air purification device.

The known devices are also often prone to overheating which reduces theefficiency of ozone production as ozone thermally degrades above about50° C. and can cause failure of the discharge electrode arrangement dueto distortion such as bowing or warping.

It is known from U.S. Pat. No. 3,654,126 to use a fluidized bed of adielectric such as sand to provide cooling to the electrodes and therebyincrease the efficiency of ozone generation. The sand is fluidized bymeans of a high rate of gas flow passing between the electrodes. Howeverthe device described in this prior publication is bulky, expensive andmechanically complex. It is thus not suitable for use in airpurification applications.

It is an object of the present invention to provide an air purificationdevice which obviates or mitigates some or all of the problems mentionedwith prior art devices.

According to a first aspect of the present invention there is providedan apparatus for purifying air comprising two electrodes having adielectric material therebetween and means for applying a potentialdifference across the electrodes, wherein the electrodes are airpermeable and the dielectric material is in the form of an airpermeable, fixed bed and wherein means are provided to provide air flowthrough one electrode, across the fixed bed of dielectric material andthrough the other electrode.

According to a second aspect of the present invention there is provideda method of purifying air comprising passing the air through an airpermeable electrode, an air permeable fixed bed of dielectric materialand a second air permeable electrode whilst applying a potentialdifference across the bed to convert oxygen in the air to ozone.

The term “fixed bed” is intended to describe a material that extendsbetween the electrodes so that it does not move in normal usage of thedevice but which is air permeable so that air may flow through gapspresent therein. The term is intended to cover inter alia a bed ofdiscrete particles, a foam, a sponge-like structure, and a bed ofelongate elements such as filaments arranged in a contactingrelationship with air gaps therebetween.

The invention causes air to flow through the air permeable electrodesand dielectric material, preferably in a direction parallel to theapplied electric field, thus substantially increasing airflow throughthe device. This means that a simple fan may be used to push air throughthe device, rather than an expensive air pump. The air purificationdevice of the present invention thus generates ozone with a high volumeof airflow, but in low concentrations, eliminating problems of exceedingtoxicity levels in “populated” areas.

Cooling of the device, which increases the efficiency of ozonegeneration, is achieved by the use of a high air flow through thedevice, which is not possible with a solid, impermeable dielectricmaterial, and without the need to fluidize a particle bed as describedin the prior art. Cooling is thus achieved without expensive and bulkyfluidization apparatus or air pumps or cooling water.

The use of the fixed bed also allows discharge to take at the points ofcontact of the dielectric (if the dielectric is in the form of discreteparticles) so that the discharge across the electrodes is uniform. As aresult, the spacing between the electrodes may be increased compared tothe prior art devices, allowing greater airflow through the device. Thefixed bed reduces level of airborne pollutants (especially particulatessuch as smoke, dust, soot, etc; aerosols; and bacteria) by electrostaticdust precipitation and by the chemical processes of ozonolysis,oxidation and sterilisation, and by the particles being burnt off due toelectrical discharge at the points of contact of the dielectric.

The electrodes may be formed of a metal gauze or mesh. Suitable metalsinclude steel and nickel.

The dielectric material may be any suitable material, but preferably hasa dielectric constant less than 100, and more preferably less than 20.The dielectric material is preferably glass. The use of a material witha reasonably low dielectric constant, such as glass, allows for costsavings over dielectric materials having a higher dielectric constant,whilst still allowing the device to be efficient enough for use in airpurification applications. Silica, alumina or another suitabledielectric (zirconia, sapphire, etc.) could be used in place of glass.Alternatively, materials such as barium titanate, which has a dielectricconstant of 1000 may be used as the dielectric material, as it may beobtained at a relatively low cost.

Preferably the dielectric material is formed of a bed of discrete bodiesin a contacting relationship. The discrete bodies are preferablyparticles, preferably regularly shaped particles and more preferablybeads. The diameter of the beads is preferably about 1 mm to 6 mm. Glasswool, chips, or extruded foam could be used in place of beads providedthat air permeability is retained and that elements of the dielectricmaterial are in a contacting relationship, although regularly spacedbeads give an advantage in that better airflow is allowed through thedielectric bed.

The potential difference applied across the electrodes may beV_(pk-pk)=10-20 kV and 10-15 kHz, although voltage such as mains at 50Hz or 60 Hz could be used.

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is an expanded, schematic cross-sectional view of an air purifieraccording to the present invention; and

FIG. 2 is a schematic perspective view of a packed bed electrodeaccording to the present invention.

Referring to the drawings, there is illustrated an air purifier 1comprising a fan 2 in a housing 3, downstream of which is a spacer 4,and retaining grids 5 and 6. A pellet bed 7 is arranged between theretaining grids such that grid 5 is upstream and grid 6 is downstream ofthe pellet bed 7. Electrodes 8, 9 are also provided with electrode 8situated between retaining grid 5 and pellet bed 7, with electrode 9situated between retaining grid 6 and the pellet bed 7.

The fan 2 and housing 3 may be of any suitable type. In experiments a240-volt fan in a square cross-section housing 3 having dimensions of120 mm×120 mm was used. The spacer 4 may be of Perspex and shouldpreferably have the same cross-sectional dimensions as the fan housing3. A spacer having 120 mm×120 mm external dimensions with a 95 mm×95 mminternal cut-out and having a thickness of 1.8 cm was successfully usedin experiments.

Retaining grids 5 and 6 are of a plastics material having the samecross-sectional dimensions as the fan housing 3 and spacer 4. Theretaining grids are to provide reinforcement to the pellet bed 7 and somust have as many holes as possible to provide a good air flow throughthe device but must also retain enough rigidity to provide the requiredstrengthening to the pellet bed 7.

Pellet bed 7 comprises two gauze electrodes 8, 9 separated by a Perspexspacer 10. The spacer 10 is of a similar shape to spacer 4, to create ahousing for pellet bed 7. Glass beads 11 are packed into the spacebetween the electrodes 8, 9 created by the spacer 10. Gauze electrode 8is of high voltage steel high transmittance gauze, although may beformed of nickel, such gauzes being available as radio frequencyinterference screens. The electrode is made using a basic size gauze of120 mm×120 mm, which is trimmed by 5 mm around each edge, the edges thenbeing sealed with epoxy. The gauze is sealed between retaining grid 5and the Perspex spacer 10 with silicone rubber compound. A high voltagecable (not shown) is attached to the gauze by solder and epoxyoverlayer. Gauze electrode 9 is similar to gauze electrode 8 except thatepoxy edging need not be applied to the gauze. A standard earth lead(not shown) is attached to the electrode instead of the high voltagecable.

The glass beads 11 are packed into the interior of pellet bed 7 formedbetween the electrodes 8, 9 and Perspex spacer 10. The beads 11 may be 6mm diameter and are preferably packed in at least two layers. Thespacing between the beads allows air to flow through the bed 7.

The ozone generator 1 would in use be surrounded by a housing having anair inlet and an air outlet, and would have a power supply and operatingswitches. These additional features are not shown in the drawings, asthey are comprised of standard components. A timer mechanism may also beincorporated in the device.

The size of glass particle used in the device is not crucial, andparticles between 1 mm and 6 mm diameter can be used successfully. Withparticles of a small size, however, the airflow through the device isrestricted. The use of glass rather than a material having a highdielectric constant does reduce the efficiency at which ozone isgenerated, but in an air quality improvement device high rates ofproduction of ozone could be extremely dangerous if someone were toenter the area being cleaned before the ozone had degraded ordissipated. Efficiency of ozone production is therefore not required tobe very high. Glass is also a cheap substance, which gives a significantcost advantage.

In order to further reduce the toxicity problems associated with ozonegeneration, a chamber could be attached to the device downstream of thepacked bed in which the reaction of ozone with airborne contaminantscould be allowed to take place. A further metallic gauze or griddownstream of the reaction chamber is used to break down the ozonebefore the air is returned to the room. This gauze could also be coatedwith a suitable material for deactivating ozone.

A coarse filter could be added to the device upstream of the fan, toremove large particles from the air.

In use, the apparatus is placed in a room or other location where theair is to be cleaned, either for the elimination of odours or to removepollutants, bacteria or other contaminants. The fan 2 is switched ontogether with the voltage supply to the pellet bed 7. The fan causes airto be pushed through the bed 7 whilst the electrodes 8,9 and the glassparticles 11 cause ozone to be generated from the air by discharge dueto a high frequency electric field being generated across bed 7, inaccordance with the known discharge principles.

The voltage applied to the device is typically V_(pk-pk)=10-20 kV and10-15 kHz. High voltage such as mains at 50 Hz or 60 Hz could also beused.

The apparatus is left running for a sufficient length of time for theozone generated to have purified the air. By air purification is meantan improvement in the quality of the air due to the removal of at leastsome of the airborne particulate matter, pollutants and odours that wasinitially present in the air. The electric discharge created in thepacked bed will also destroy some particulate matter that may be presentin the air either by burning it off or by means of electrostaticprecipitation onto the dielectric particles, which will further improvethe air quality.

As the air flows through the gauze electrodes rather than having to flowthrough the gap between the electrodes, the airflow rate through thedevice is significantly improved over the prior art devices. The packedbed of dielectric means that electric discharge takes place at thepoints of contact between the dielectric particles, making the dischargevery uniform and allowing the spacing between the electrodes to beincreased.

What is claimed is:
 1. An apparatus for purifying air comprising: (i) apair of opposed, spaced air permeable electrodes, (ii) an air permeablefixed bed which is comprised of discrete particles of a dielectricmaterial having a dielectric constant of less than 20, and which extendsbetween the electrodes, said particles being in contacting relationship,(iii) means for applying a potential difference across the electrodes toprovide an electric field in a direction between the electrodes, and(iv) means for providing an air flow through one electrode, across thefixed bed of dielectric material and through the other electrode.
 2. Anapparatus according to claim 1, wherein the means to provide air flowthrough the apparatus is adapted to provide air flow in a directionparallel to the electric field generated to the applied potentialdifference.
 3. An apparatus according to claim 1, wherein the means forapplying potential difference across the electrodes is adapted toprovide a potential difference of V_(pk-pk)=10-20 kV at 10-15 kHz.
 4. Anapparatus according to claim 1, wherein the means to provide air flowthrough the apparatus is a fan.
 5. An apparatus according to claim 1,wherein the electrodes are formed of a metal gauze or mesh.
 6. Anapparatus according to claim 5, wherein the metal gauze or mesh iscomprised of steel or nickel.
 7. An apparatus according to claim 6,wherein the dielectric material is glass.
 8. An apparatus according toclaim 1, wherein the fixed bed of dielectric material comprises a bed ofdiscrete particles, a foam, a sponge-like structure, or a bed ofelongate elements arranged in a contacting relationship with air gapstherebetween.
 9. An apparatus according to claim 8, wherein thedielectric material comprises discrete particles, such discreteparticles being regularly shaped beads.
 10. An apparatus according toclaim 9, wherein the beads have a diameter of 1 mm to 6 mm.
 11. A methodof purifying air comprising passing the air through a first airpermeable electrode, through an air permeable fixed bed of discreteparticles of a dielectric material having a dielectric constant of lessthan 20 and through a second air permeable electrode whilst applying apotential difference across the electrodes to generate an electric fieldin a direction between the electrodes.
 12. A method according to claim11, wherein the air is passed through the electrodes and the bed ofdielectric material in a direction parallel to the electric fieldgenerated between the electrodes.
 13. A method according to claim 11,wherein the potential difference is applied across the electrodes at avoltage of V_(pk-pk)=10-20 kV at 10-15 kHz.
 14. A method according toclaim 11, wherein the air is passed through the electrodes and the bedof dielectric material by means of a fan.